There have been significant advances in the microminiaturization of electronic systems and this has spurred the creation of space-saving switchmode conversion techniques for the design of highly efficient power processing equipment. An excellent overview of the state of the art is presented in the book "Modern DC-to-DC Switchmode Power Converter Circuits", by Rudolf P. Severns and Gordon E. Bloom, Van Nostrand Reinhold Company, 1985.
A continuing goal of electrical engineers has been the goal of reducing the physical size and parts count of switchmode power converters. The blending together or combining of inductors and transformers into single physical assemblies with little of no compromise in the desired conversion characteristics has been termed "integrated magnetics". One important reason for this effort has been the fact that inductors and transformers are major contributors to the total cost, weight and size of a converter system. Magnetic integration, if properly executed in the design of power converters, can bring added benefits in electrical performance, such as reduced stress on the components or lower ripple currents on input and output power lines. Chapter 12 of the textbook previously cited provides an excellent foundation on the subject of integrated magnetics. Two other pertinent publications are: Core Selection for an Design Aspects of an Integrated--Magnetic Forward Converter, by Ed Bloom, IEEE Applied Power Electronics Conference, New Orleans, Conference Proceedings, April 1986, pages 141-150; New Integrated--Magnetic DC-DC Power Converter Circuits and Systems, by Ed Bloom, IEEE Applied Power Electronics Conference, San Diego, Conference Proceedings, Mar. 2, 1987, pages 57-66. Some U.S. Patents on the subject of integrated magnetics and converters are: 4,675,797; 4,688,160; 4,675,796; 4,561,046; 4,538,219; 4,355,352; 4,262,328; 4,257,087; 3,694,726; 3,553,620.
However, the process of magnetic integration is not simple. For the most part, integrating magnetic elements of a converter does not add to the many design aspects and difficulties with which an engineer must contend during the converter's development phase. However, it does place additional burden on the designer to properly specify the performance of the integrated magnetic elements and a burden on the manufacturer to insure a consistent product which can be more complex than a simple transformer or inductor assembly. For example, there are some subtle electrical problems which arise when coupling inductors together on a common magnetic core. Imbalances in the required turns ratio relationships between the inductor and transformer windings can produce circulating currents in filter networks, resulting in excessive ripple current. High power loss in filter capacitors can result in converter damage and even destruction of the filter elements. Thus, integrated magnetics, as applied to switchmode power converter circuits, is a concept which is not straight-forward or easy to apply in practice.
Another example is the start-up circuit which is used to put the converter in operation. Heretofore, separate drive transformers and special solid-state components have been used. Such a design adds weight and does not contribute to an optimal use of equipment and components. U.S. Pat. Nos. 4,063,307; 4,694,386; and 4,695,936 are three examples of circuits that have been used to start or trigger a converter.
Any advancement that can be made and any teaching in this highly complex subject area will be welcomed by those skilled in the art, and any practical embodiment of these teachings will advance the state of the art. This is particularly true if subtle design considerations are highlighted and circuits are presented which result in unexpected improved performance or operating characteristics, especially when only a few components are added relative to conventional converter designs.